Dielectric ring lasers using waveguiding

ABSTRACT

There is disclosed a ring laser in which mode-controllable oscillations occur in a dye-doped dielectric film along an axis wrapped around a lower-index dielectric. Output coupling is typically achieved by fringing field coupling to a nearby dielectric body of higher index.

United Sta y Q W 1111 3,725,809

Ulrich et al. 1 Apr. 3, 1973 [54] DIELECTRIC RING LASERS USING [56]References Cited WAVEGUIDING UNITED STATES PATENTS [75] Inventors:Reinhard Ulrich, Matawan; Heinz Paul Weber, Middletown, both of3,585,521 6/1971 Ni 3,222,615 12/1965 3,197,715 7/1965 [73] Assignee:Bell Telephone Laboratories, Incor= 3,140,451 7/1964 porated, MurrayHill, Berkeley Heights, NJ. Primary Examiner-Ronald L. Wibert AssistantExaminer-V. P. McGraw [22] Filed 1971 Attorney-R. J. Guenther and ArthurJ. T orsiglieri 21 Appl. No.: 131,296

[57] ABSTRACT [52] US. Cl. ..33l/94.5, 350/96 WG There is disclosed aring laser in which mode-control- [51] Int. Cl ..H0ls 3/00, G02b 5/ 141lable oscillations occur in a dye-doped dielectric film [58] Field ofSearch ..33l/94.5; 350/96 WG along an axis wrapped around a lower-indexdielectric.

Output coupling is typically achieved by fringing field coupling to anearby dielectric body of higher index.

4 Claims, 5 Drawing Figures N LASER PUMP SOURCE 12 3725899 .1; I UR IN331/94w5C e PATENTEnA'PRs 197a SHEET 1 UF 5 FIG.

N2 LASER PUMP SOURCE lNl EN TOPS R. UL RICH H. P; v WEBER'PATEIITFIIIIFRGI I375 I NTENSITY (ARB. UNITS) INTENSITY (ARB. UNITS o lllllll l llllll l sum 2 [1F 3 FIG. 2A

WAVELENGTH WAVELENGTH PAII-jIIIIIIIPIIs I075 3,725,809

SHEET 3 III 3 FIG. 3

NEODYMIUM-DOPED THlNa-lFILM FREQUENCY- SELECTIVE PRISM UTILIZATION %TAPPARATUS I2 FIG. 4

I I PUMPING ]k-SII-I%I 4| A I 47 DEVICES BACKGROUND OF THE INVENTION inthin films have been largely unsuccessful. While the 1 reasons for thisare various, some of the more important reasons are related to thedifficulties in establishing the required feedback of the guided light.For example, high optical losses are typically associated with providinga laser resonator for laser action in the thin film. As a consequence,the laser sources used in typical work with optical thin films are stillrelatively bulky laser sources which limit the degree of miniaturizationand compactness of the integrated circuit.

Furthermore, quite apart from optical integrated circuits, it would bedesirable to have sources of laser light that are not dependent upon thecritical alignment of the typical optical resonator. Eliminating theresonator alignment problems would make it feasible to use lasers formany practical uses in small businesses of many types. For example, suchlasers could be used by automobile service station men or others fortesting automobile exhaust for incomplete combustion or air pollution.They could also be used by a repairman in the field for making manykinds of tests.

SUMMARY OF THE INVENTION Our invention is based on our discovery of ringlaser action in a thin film of a transparent dielectric medium mixedwith a laser-active medium such as a dye.

A feature of our invention is directed to a ring laser in whichmode-controllable oscillations occur in an active dielectriclight-guiding film along an axis wrapped around a lower-indexdielectric. The feedback for laser oscillation occurs by continuingpropagation of the stimulated light along that axis, which closes uponitself. Output coupling is typically achieved by fringing field couplingto a nearby dielectric body of higher index.

In a specific successful embodiment of our invention, the ring laser isformed by a single-mode, lightguiding thin film of rhodamine 6G dopedpolyurethane coated on the surface of a cylindrical glass rod.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. I is a partially pictorial and partially block diagrammaticillustration of an illustrative embodiment of our invention;

FIGS. 2A and 2B show curves useful in explaining the observed operationof the embodiment of FIG. 1;

FIG. 3 shows a modified embodiment of our invention, useful forobtaining a frequency-selective output; and

FIG. 4 shows a modified embodiment useful in thinfilm integrated opticalcircuits.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT Our observation of the stimulatedemission of coherent radiation in thin-film light guides offers thepossibility of constructing lasers that are completely compatible withoptical integrated circuits. In the embodiment of FIG. I are shown thebasic elements of the apparatus in which we have obtained what webelieve to be one of the first successful laser oscillations in alight-guiding thin film. While the embodiment of FIG. 1, to be describedherein, is not necessarily the precise form that would be desirable inan optical integrated circuit, its performance should be obtainable inequivalent modified embodiments. We have observed gains as high as dB/cmin single-mode light-guiding thin films in the embodiment of FIG. 1.

In FIG. I, the light-guiding thin film 11 is a polyurethane film dopedwith rhodamine 6G dye and pumped by a pulsed nitrogen laser source 12.

The doped light-guiding film 11 is applied on the surface of acylindrical glass rod 13. In this way, a closed optical path isestablished along any circumference of the rod. This closed pathprovides the feedback required for laser action. The laser action willtake place along a given circumference if the gain along thatcircumference exceeds the round-trip loss. Since the material ishomogeneous along this path without surfaces intervening in the path,very low losses can be obtained.

In the embodiment of FIG. I, the laser light generated in the film iscoupled out for utilization by a prism-film coupler of the typedisclosed and claimed in the copending patent application of I. K. Tien,Ser. No. 793,696, filed Jan. 24, I969, assigned to the assignee hereof,and now U.S.Pat. No. 3,584,230, issued June 8, l97l. The prism-filmcoupler is implemented by a flint glass isosceles prism 14 which isbrought substantially into contact with film 11, but in practice isseparated by a small air gap therefrom because of dust particles andminor surface imperfections in the adjacent surfaces of film l1 andprism 14.

Two output beams are obtained because of the directional nature of thecoupling through prism 14. These output beams correspond respectively toclockwise and counterclockwise oscillations of the laser light about thecircumference of rod 13 in film 11 as viewed along the axis of rod 13 ineither direction. Either of the two output beams can be used in asuitable utilization apparatus (not shown), which could be a sample ofgas or other material to be analyzed and a suitable detector orspectrometer.

The following specific details apply to our successfully operatedexperimental model of the embodiment of FIG. 1. The pulsed nitrogenlaser pump source operated at a wavelength of 3371 A. and, because ofits transverse discharge configuration, emitted a sheetlike beam ofrectangular cross section that pumped a narrow (approximately 0.2millimeter high) circumferential strip of the film 11. The diameter ofthe rod in this experiment was 5 millimeters. At peak intensity, a pumppower of approximately I5 kilowatts in 10 nanosecond pulses was incidenton the laser rod. The pump intensity thus provided was approximately l.5megawatts per square centimeter. Nevertheless, it

should be apparent that many other pump sources other than the pulsednitrogen laser source 12 could be used and that transverse dischargelasers are also not essential to our invention, even though it isdesirable to form the pumping laser beam into a sheet illuminating acircumferential strip of the film 11.

The polyurethane film 11 had a refractive index of 1.55. It can becoated on the surface of the Pyrex glass rod 13 of index 1.47 byconventional techniques like dipping, spraying, painting. The rhodamine6G dopant must be maintained uniformly in the film. This can be done bymixing the polyurethane before its polymerization with the rhodamine orwith a solution of the rhodamine in a suitable organic solvent likeethanol. In our successful experiments we have found a desirableconcentration of the rhodamine 6G dye to be between 10 and 10" 1 molesper liter in the film and that a desirable film thickness is about 0.8micrometer.

While our prism 14 had an index of 1.64, it should be apparent that manyalternatives for output coupling are available. For example, diffractiongrating-type couplers are also feasible and may be coated onto thesurface of film 11 in a limited region thereof. The latter type ofcoupler may be of the general type disclosed in the copending patentapplication of H. W. Kogelnik, Ser. No. 67,857, filed Aug. 28, 1970.

In the operation of the embodiment of FIG. 1, we observed gains as highas 100 dB/cm. These gains were determined by measuring the amplificationof the spontaneous emission, as described in the copending patentapplication of R. F. Leheny and K. L. Shaklee, Ser. No. 112,237, filedFeb. 3, 1971, and assigned to the assignee hereof. The two output beamsare well collimated in the vertical direction because of the narrowheight (approximately 0.2 millimeter) of the pumped region. In thehorizontal direction they are spread out over an angle of approximately1 because of the finite spectral width of the laser light and thecurvature of the film. The curvature of the thin film 11 results in anapproximately Gaussian intensity distribution in the horizontal plane ofthe output beams. Suchi Gaussian distribution is the most desirable formany applications, such as those using nonlinear optical devices, and istherefore a particularly attractive feature of the prismfilm couplingscheme when used with the thin-film ring laser.

The reason for the Gaussian intensity distribution lies in theparticular nonuniform width S(x) of the coupling gap between the base ofthe prism 14 and the film 11. The direction x is indicated in FIG. 1 andis tangent to the cylinder at the center of the coupling region. The gapwidth can be written as S(x)=S,,+x/2R. (I)

Here S is the minum width of the gap, and R is the radius of curvatureof the thin film 11. Using Equation (1) and the mathematical formalismgiven in the Jourml of the Optical Society of America, Vol. 60, pages1337-1350, it can be shown that the output beams have a Gaussian crosssection in the horizontal plane. It is assumed here that only a smallfraction of the circulating power in the laser is coupled out. The waistradius w of the beams, measured at the prism base along the x direction,is approximately Here A is the wavelength of laser oscillation, n is therefractive index of the medium outside of the film 11, and N is theeffective refractive index of the film.

Our experiment showed that the gap width S between the prism and film,while not measured, could be optimized by adjusting the pressure betweenthe prism and the laser rod to obtain maximum output coupling.Typically, this gap width is less than one wavelength of the light.

It may appear that there should also be radiation loss caused by thecurvature of the thin-film guide 11. Such radiation would leave thelaser in a tangential direction and would be highly collimated in theplane of the ring. Our quantative estimates show, nevertheless, thatthis output coupling mechanism is entirely negligible. Specifically, itis less than 10 per round trip for the 2.5 millimeter radius ofcurvature of rod 13. Such losses will be below the above-stated levelfor radii of curvature as small as 0.5 millimeter.

In our experimental arrangement of FIG. 1, only a small fraction of thepump intensity supplied to the film 11 is actually used for pumping thelaser because the single-mode film 11 is so thin that it absorbs onlyvery weakly. In addition, at both ends of the pump region the angle ofincidence of the pump light with respect to the normal to the film is solarge that a considerable fraction of the pump radiation is reflected.We estimate that about 1 kilowatt of the pump light is absorbed in ourarrangement. The measured output of intensity was about watts in each ofthe two emerging beams. It appears, therefore, that the net efficiencyof this laser must be quite high. It is apparent that the overallefficiency of the laser could be considerably improved and the totalpump power requirements reduced by providing means for passing the pumplight repeatedly through the film or by pumping on a stronger absorptionband of the dye dopant of film 11. For the rhodamine 6G, this would beachieved by pumping in its green absorption band, where its absorptioncross section is more than an order of magnitude larger than at 3371 A.

In FIG. 2A is shown a curve 21 of the relative intensity of the laseroutput as a function of wavelength between about 5800 A and 6600 A. Thespectrum peaks at about 6200 A, as shown by curve 21 of FIG. 2A. Thisrelatively long wavelength is a result of the heavy doping of 8'10mol/liter in the film. For lighter doping, the spectrum shifts toshorter wavelengths. The bandwidth of the laser at half intensity is I10 A. This is much narrower than the width of the spontaneous emissionof several hundred Angstroms, as measured in the same host material.Such a narrowing of the output spectrum is also characteristic ofsuper-radiant emission. Nevertheless, a direct proof of positivefeedback within the pumped region of film 11 around rod 13 is obtainedfrom our observation of the individual circumferential modes of laseroscillation having a frequency spacing characteristic of the ring laserresonator. Our observation of these modes is illustrated by curve 22 ofFIG. 2B. In curve 22 of FIG. 28 we have plotted the relative outputintensity in arbitrary units similar to those of FIG. 2A but on a muchmore finally resolved frequency scale centered about 6204 A, and

have marked the individual mode peaks by small arrows above curve 22.These arrows may be seen to be equally spaced by about 0.555 A. Theirmode spacing may be determined analytically by analogy to thelongitudinal mode spacing of Fabry-Perot laser resonators using twoseparate reflectors. The theoretical mode spacing is A). X /rrdN 3 whered 2R is the diameter of the rod, and N, c/v group is the effective groupvelocity index of the guide.

Corresponding to the transverse modes of a Fabry- Perot laser resonatorare the radial and axial modes of the thin film on the outside of therod. The radial modes are the well-known TE, and TM,, modes of athin-film guide. Our films permitted propagation of only the fundamental(m=) modes in the radial direction. In the axial direction, there are norelevant boundaries to define axial modes in the absence of pumping.However, in the pumped rod the strip of high gain is bordered at bothsides by unpumped, slightly lossy regions of the film defining the modesin axial direction, the fundamental one having the lowest loss.

From the observed uniform distribution of the light intensity across theoutput beams and from the diffraction limited divergence, we concludethat we did obtain laser oscillation in the fundamental transverse(radially as well as axially) mode of the ring laser. However, anynonuniformity of the thin-film guide, e.g., a dust particle, will spoilthe ideal modes described above. When pumped near such nonuniformity,the laser output beams broke up into a series of several horizontallines. They covered a wider vertical angle than the fundamental mode.This indicates oscillation of higher order axial modes.

For the rod of mm diameter and a group velocity index of the guide of N,1.584, the longitudinal mode spacing is only AA 0.154 A, which isdifficult to resolve with a grating spectrometer. In order to increasethe mode spacing to a measurable value, we pumped a thinner rod of 1.397millimeter diameter having a theoretical mode spacing of Alt 0.551 A.The resolved output spectrum of this rod is the one shown in FIG. 2B.The individual modes are clearly separated. Their measured spacing is0.555 t 0.005 A, in good agreement with the theoretically expectedvalue. In the thin rods the axial mode structure was not as clean as inthe 5mm thick rod. We believe that this is the reason for the weaksecondary set of lines observed in FIG. 2B. In order to prove that wehave seen the longitudinal modes of the ring resonator and not thespectrum due to any structure of the rhodamine 6G molecule, we pumpedanother thin rod of 1.054 millimeter diameter. This one was doped lessheavily and had its peak output at 6050 A. Its theoretical mode spacingis AA 0.696 A; we measured 0.71 i 0.02 A.

As in other host materials, the rhodamine 6G showed irreversiblebleaching, resulting from the pump radiation. This effect limited thelaser operation to about 10 10" shots at a given spot on the laser rod.Laser action started again when the pump beam was moved to a fresh spoton the rod.

The embodiment of FIG. 3 is modified from that shown in FIG. 1 in orderto provide a frequencyselective laser oscillation about the axis of therod 33 and a corresponding output of coherent light of the selectedfrequency to the utilization apparatus 32. A narrowband orsingle-frequency output is desirable for many uses, including that inwhich the utilization apparatus 32 could, for example, be a modulator inan optical communication system.

The rod 33 is a transparent dielectric of the type described above inFIG. 1 and the film 31 coated upon the lateral surface thereof may be adye-doped polymer or other dielectric film of the type disclosed abovefor the embodiment of FIG. 1. Also, as in FIG. 1, the laser pump sourceis selected to have a frequency permitting at least a portion of thepump energy to be absorbed by the active medium in film 31.

Two principal differences from the embodiment of FIG. 1 stand out in themodified embodiment of FIG. 3; first, the output coupling isillustratively provided by a diffraction grating 34 ruled in the surfaceof film 31 in a direction transverse to the oscillation path about theaxis of cylinder 33. An output coupling prism thereby becomesunnecessary. Second, a frequency-selective prism 35 is provided in theclosed oscillation path and, indeed, may .be replicated at a number ofother axial positions for other possible oscillation paths in film 31,as shown. Each such frequency-selective prism 35 includes the triangularprism elements 36, 37 and 38. The prism elements 36 through 38 areillustratively regions of differing thicknesses both with respect to thesurrounding area of film 31 and with respect to their neighboring prismelements. The differing thicknesses provide the differing phasepropagation constants in the three prism elements which are advantageousfor a high degree of frequency selectivity. The combined effect of thethree prisms 36, 37 and 38 is that they provide a closed feedback pathonly for one selected wavelength of light. The light of differentwavelengths is deflected by the prisms into spiraling paths on the rod33, thus preventing feedback for these undesired wavelengths. Theoperation of such prisms has been described in the copending patentapplication of R. J. Martin and R. Ulrich, Ser. No. 835,484, filed June23, 1969 and assigned to the assignee hereof, now U.S. Pat. No.3,614,198, issued Oct. 19,1971.

In the operation of the embodiment of FIG. 3 for the illustrative casein which the active medium is a rhodamine 6G dye in a polyurethane film31, the broad oscillation band obtainable when source 13 suppliespumping power above threshold, makes such frequency selection desirable.Even in the case of active media with oscillation bandwidths less thanthat of dyes, such frequency selection is still advantageous. Forexample, the active medium could be trivalent neodymium ions distributedthrough a glass film 31 of higher refractive index than a central glassrod 33. The thickness of film 31 would be chosen to be about onehalf-micron, depending on the index difference, to provide goodtransverse mode control of the neodymium laser oscillation, and thefrequency-selective prism 35 would supply sufficient selectivity forsingle-axial-mode oscillations near 1.06 micrometers. Such complete modeselectivity of the 1.06 micrometer neodymium laser oscillations has notbeen achievable in any simple and reliable apparatus heretofore.

It is readily apparent in the embodiments of both FIGS. 1 and 3 that, ifone were to take a thin cross-sectional slice of the cylindricalstructure, an essentially similar ring laser structure is stillobtained. The visualization of this modification illustrates that thepresent invention is compatible with thin-film optical devices.

Thus, a thin-film ring laser suitable for supplying laser light to otherthin-film optical devices is shown in the modified embodiment of FIG. 4.

In FIG. 4 the cylindrical rod of the previous embodiments is replaced bya substrate 42 on a major surface of which the thin-film active medium41 is deposited in a ring-like shape thereon about an unoccupied regionof the substrate. Here, the thinnest dimension of the film is orthogonalto the radius of curvature. To pro vide output coupling, anotherthin-film region 43 is deposited on substrate 42 with a separation ofthe order of magnitude of one oscillation wavelength therefrom. Laserpumping light is coupled into the ring 41 by a coupling prism 44 into aguiding path 45, which may or may not include active medium, whichmerges with ring 41 in a direction tangent thereto to provide continuousguiding of the pumping light. The pumping light is supplied from pumpinglaser source 46, which could be essentially similar to the pumpingsources employed in the previous embodiments, with the exception thatthe pump light must be focused into the prism 44 at a suitable angle andwith a suitable width to be coupled into the guide region 45 in aphase-matched manner. The use of a phase-matched prism film coupling isessentially the same as that disclosed in the above-cited copendingallowed application of P. K. Tien.

In the embodiment of FIG. 4, the utilization apparatus would typicallybe thin-film devices 48 which may be disposed in the thin-film region43. These devices 48 could include modulators, optical guides,amplifiers and detectors. Also, while the devices 48 are showncompactly, it should be understood that the transmission of the light insuch devices could occur over a substantial distance.

In the operation of the embodiment of FIG. 4, which is otherwise similarto that of the preceding embodiments, it may be desirable to obtainmaximum utilization of the active medium by providing unidirectionaltraveling wave oscillations about the ring instead of the standing waveoscillations that are the resultant of oscillations traveling in bothdirections about the ring. To this end, one of the directions ofoscillation is quenched by reflecting a portion of that oscillationcoupled into region 43 back upon itself into ring 41. The otherdirection of traveling wave oscillation then predominates and grows atthe expense of the other direction of oscillation.

In all of the foregoing embodiments it should be understood that thering-like path in which oscillations occur need not be circular; but itshould have at all points a sufficiently large radius of curvature sothat guiding is maintained. For example, an elliptical oscillation pathappears to be feasible. As a corollary, the rod 13 of FIG. 1 need not becylindrical; and, even'ifelliptical, need not have two symmetricallydisposed foci. In the latter case, the rod does not have a uniquelydefinable axis. Thus, in the claims hereafter, the word'- axis" shouldbe read to be any axis about which the surface of the rod curves.Moreover, in such a case, for films 11 providing sufficiently tightoptical guiding, a sufficient restriction upon the shape of the rodsurface is that the first derivative of its curvature be continuous.This condition provides a smooth surface of the type suitable foroptical guiding. Similar restrictions on the curvature of theoscillation path can be stated for the other embodiments. The capabilityof the guiding thin film to guide the light around a point of maximumcurvature in such a case is determined by the smallness of the dimensionof the film along the radius of curvature.

Various techniques can be used to produce a lightguiding film containingactive molecules or ions. In our experiments we used films ofpolyurethane. These films were produced by mixing a solution of themonomeric material with a suitable chemical reagent catalyzing thepolymerization process. In addition, the dye is added to this solution.The solution is applied to the surface of the glass rod by dip-coating,although it could be done also by spraying or brushing the solution onthe rod. The rod is then held in a vertical position, and uponevaporation of the solvent, a thin, soft film is left. Thepolymerization is achieved by gently heating (e.g., to C) the rod for alength of time (e.g. 1 hour). The curing temperature must be low enoughso as not to cause thermal decomposition of the dye. The thickness ofthe film thus produced depends on the solid content and the viscosity ofthe solution, and on the temperature of its application. Typical valuesare a solid content of 10 percent by volume, and a viscosity of 20 cps.

It is clear that this method of fabrication is possible for a greatvariety of other polymer films and other dyes, for example films ofpolystyrene, polyester, polymethyl methacrylate and epoxies. Anotherpossible dye is sodium-fluorescein giving a laser operation in thegreen, or any soluble chemical compound containing trivalent neodymium.

The light-guiding film need not be an organic polymer. Films of goodoptical quality can be prepared by wet chemical methods, producingmixtures of silicon dioxide and lead oxide of a gel-like consistency.Upon mild heating (100 C) these films become hard and glassy. Here againthe dye is mixed with the solution before it is applied on the rod.

The light-guiding filmsmay alternatively be produced by vacuumdeposition or sputtering of a glassy or crystalline material on the rodor other substrate. In these cases, one can, for example, bring ions ofneodymium into the film either by simultaneous evaporation or sputteringof a suitable neodymium compound (e.g. Nd O or by a subsequent diffusionprocess in which the completed film is immersed into a hot, concentratedsolution containing trivalent neodymium.

lclaim:

l. A ring laser in which coherent optical oscillations occur whensupplied with pumping energy, comprising a first body including anactive medium and forming a closed, continuously-curved ring-likefeedback path for said oscillations about a central axis, a second bodyextending through said axis, said second body having a lower index ofrefraction than said first body and a smooth continuously curved majorsurface adjoining said first body along said path, said first bodyhaving a dimension transverse to said path and orthogonal to said majorsurface of the order of an expected oscillation wavelength to waveguidesaid oscillations, means for supplying said pumping energy to said body,and means for coupling a portion of said oscillations from said bodythrough a major surface thereof.

2. A ring laser according to claim 1 in which the second body comprisesa dielectric rod the axis of which is the central axis and the firstbody extends about and in continuous contact with the lateral surface ofsaid rod to form the closed feedback path about the axis of said rod.

3. A ring laser according to claim 1 in which the first body includes inthe closed feedback path at least one region in which the transversedimension of the order of the oscillation wavelength is varied alongsaid path to select a value of said oscillation wavelength.

4. A ring laser comprising a rod having a smooth, continuously-curvedlateral surface and comprising a transparent dielectric material, anddeposited on said rod about the axis thereof a mixture of a dielectricmedium and an active medium forming a closed, continuously-curvedring-like feedback path for optical oscillations about the axis of saidrod, said mixture having a smooth, continuously-curved major surfacecontinuously in contact with said lateral surface along said path andhaving a'thickness orthogonal to said major surface of the order ofmagnitude of an optical emission wavelength of said active medium towaveguide said oscillations with respect to the curvature of saidcontinuously-curved feedback path, and means for coupling optical energyout of said mixture through a major surface thereof.

2. A ring laser according to claim 1 in which the second body comprisesa dielectric rod the axis of which is the central axis and the firstbody extends about and in continuous contact with the lateral surface ofsaid rod to form the closed feedback path about the axis of said rod. 3.A ring laser according to claim 1 in which the first body includes inthe closed feedback path at least one region in which the transversedimension of the order of the oscillation wavelength is varied alongsaid path to select a value of said oscillation wavelength.
 4. A ringlaser comprising a rod having a smooth, continuously-curved lateralsurface and comprising a transparent dielectric material, and depositedon said rod about the axis thereof a mixture of a dielectric medium andan active medium forming a closed, continuously-curved ring-likefeedback path for optical oscillations about the axis of said rod, saidmixture having a smooth, continuously-curved major surface continuouslyin contact with said lateral surface along said path and having athickness orthogonal to said major surface of the order of magnitude ofan optical emission wavelength of said active medium to waveguide saidoscillations with respect to the curvature of said continuously-curvedfeedback path, and means for coupling optical energy out of said mixturethrough a major surface thereof.